1. Field of the Invention
The present invention relates to a technique for increasing a circuit efficiency and reducing a cost in a DC voltage converting circuit of a flyback type.
2. Description of the Related Art
Referring to a DC voltage converting circuit (a so-called DC-DC converter) for converting a DC input voltage into a desirable DC voltage, various configurations have been known.
For example, in the application of a discharge lamp (such as a metal halide lamp) to a lighting circuit, there is a structure of a flyback type as a DC-DC converting circuit for converting a DC input voltage applied from a DC power source into a desirable DC voltage. In other words, a semiconductor switching element is connected to the primary winding side of a transformer for conversion to carry out the ON/OFF control of the element, and a rectifier diode and a smoothing capacitor are provided on the secondary side of the transformer.
Referring to the operation of the flyback type circuit, for example, control is carried out in such a manner that the switching element is turned ON before energy stored in the transformer is completely discharged to the secondary side of the transformer in case of a current continuation mode (that is, the switching element is turned ON before the secondary current of the transformer reaches zero ampere).
Referring to the current continuation mode, there is the following problem of an electric efficiency (a rate of an output power to an input power).
First of all, in the case in which attention is paid to a power loss in the reverse recovery time of a rectifier diode provided on the secondary side of the transformer, the switching element is turned ON while a current flows to the diode. Consequently, the diode is brought into a backward bias state so that a power loss is generated in the reverse recovery time. The total amount of the loss in the reverse recovery time of the diode is proportional to a switching frequency. In the current continuation mode, therefore, the power loss is increased when the frequency is raised.
Moreover, the primary current of the transformer is generated when the switching element is turned ON. Consequently, a turn ON loss of the element is generated.
The invention has an object to cope with an increase in a switching frequency and to enhance an electric efficiency, and to prevent a cost from being increased remarkably in a DC voltage converting circuit having a flyback type structure.
The invention provides a DC voltage converting circuit having a flyback type structure, wherein in case of a current boundary mode in which control is carried out in such a manner that a switching element is turned ON when energy stored in a transformer is completely discharged to the secondary side of the transformer (that is, the switching element is turned ON when the secondary current of the transformer reaches zero ampere), the current of a rectifier diode reaches zero ampere and the switching element is then turned ON. In consideration of the fact that a power loss is not generated in a reverse recovery time and the highest electric efficiency can be obtained in the current boundary mode in case of a comparatively high switching frequency (for example, several hundreds kHz or more), the following structure is provided in order to solve the problem.
To provide control means for carrying out control in such a manner that a transformer stores energy while a switching element provided on the primary side of the transformer is set in an ON state and the energy is output from a secondary winding while the switching element is set in an OFF state, and the switching element is turned ON when the energy is completely output from the secondary wiring.
To detect a duration from a time that the secondary current of the transformer reaches zero to a time that the primary current of the transformer is started to flow and to change a switching frequency by the control means such that the duration is shortened.
According to the invention, therefore, the switching frequency is controlled such that the duration from the time that the secondary current of the transformer reaches zero to the time that the primary current of the transformer is started to flow is caused to approximate to zero. Therefore, it is possible to implement driving control in the current boundary mode, and furthermore, it is not necessary to provide a high-speed element therefor.